This invention relates in general to optical communication systems, and in particular to a system and method for detecting a condition in an optical device.
Optical communication systems, in particular long-haul networks of lengths longer than 600 kilometers, inevitably suffer from signal attenuation due to a variety of factors including scattering, absorption, and bending. To compensate for losses, repeaters are typically placed at regular intervals along the optical transmission path. Each repeater boosts the input optical signal to compensate for accumulated transmission losses. Initially, this function was accomplished solely by regenerators, which convert optical signals into electrical form and then back to optical form in order to amplify, reshape, retime, and re-transmit the optical signal. The advent of reliable and low cost optical amplifiers has largely obviated the need to make such optical-electrical-optical conversions, although longer spans may still require such conversions depending on the amount of signal degradation.
Optical amplifiers include rare earth doped fibers such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers. An EDFA operates by passing an optical signal through an erbium-doped fiber segment, and xe2x80x9cpumpingxe2x80x9d the segment with light from another source such as a laser. The pumping energy may be provided at 1480 nm or 980 nm for an EDFA, which corresponds with the absorption peaks of erbium. Raman amplification occurs throughout an optical transmission fiber when the transmission fiber is pumped at an appropriate wavelength or wavelengths. Gain is then achieved at a longer wavelength through the process of Stimulated Raman Scattering.
To measure the performance of repeaters containing optical amplifiers, optical communication systems may employ a line monitoring system (LMS). The line monitoring system may include line-monitoring equipment (LME) located in the terminal stations and loop-back paths in the repeaters and terminals. The loop-back paths (hereinafter loop-back paths) optically couple two fibers of a fiber pair (one in each direction of transmission) such that a portion of the optical signal originating at a transmitting terminal and being transmitted on one of the fibers of the pair is looped back and coupled into the fiber that is transmitting in the reverse direction back toward the transmitting terminal. The fundamental quantity measured by the LME is the round-trip loop gain between the LME and each terminal and repeater loop-back path on a fiber pair. Through routine analysis of the measured loop gains compared to a baseline loop gain at normal operating conditions, the LMS can be used to detect changes in the performance of the portion of the system spanned by the monitored repeaters and terminals over time.
The difference between the baseline loop gain levels and the measured loop gain levels is typically referred to as the loop-back signature. For example, under operating conditions, measured loop gains may be determined for each of the amplifier pairs in the sequence in which the amplifier pairs are encountered along the transmission path. That is, a first data point would represent the loop gain from the LME to the first amplifier pair, and a second data point would represent the gain from the LME to the second amplifier pair, and so on. The difference between the baseline curve and the measured curve is a representation of the loop-back signature.
An ideal signature is a straight horizontal line running through a gain change of 0 dB, indicating that all the loop gain measurements from the amplifier pairs agree exactly with the pre-established baseline. In practice, however, system noise and other transmission variations will normally occur. As a result, a nominal signature will typically have a random shape within some pre-established window about the zero line defining a nominal band of acceptability. Extreme faults, such as fiber breaks and other problems that result in immediate loss of service, will typically result in a signature shape with one or more points of the signature outside of the pre-established window. However, there is a class of other faults and conditions, which typically would not be expected to result in any discernable differences that could be detected by the LME. For instance, the failure of a redundant electrical power supply or a rise in temperature beyond the expected limits in an area housing the optical device in a terrestrial system would not otherwise be detected by the LME until it resulted in some equipment fault.
Accordingly, there is a need for a system and method that overcomes the deficiencies of the prior art and allows for signaling of a variety of faults and conditions for an optical device, such as a repeater, that may otherwise not be detected by the LME.
An optical device consistent with the invention includes first and second transmission paths, and a loop-back path. The loop-back path couples a portion of an optical signal from the first transmission path to the second transmission path as a loop-back signal, and includes at least one optical attenuator configured to attenuate the loop-back signal in response to at least one detected condition. The loop-back signal may be detected by the LME where the attenuation imparted by the attenuator is interpreted as an indication of one or more detected conditions in a class of conditions associated with the attenuator setting.
A method of monitoring an optical device consistent with the present invention includes detecting a condition in the optical device, attenuating a loop-back signal on a loop-back path in said device in response to the detected condition, and detecting the attenuated loop-back signal.